Multi-filament composite superconductor with transposition of filaments

ABSTRACT

A multi-filament superconducting composite composed of a plurality of segments that is intrinsically stable. The individual segments are multi-filament composites that have been twisted after assembly and mechanical working and thereafter subsequently deformed. The deformed segments can be triangular or rectangular and are assembled into a second composite. After assembly the second composite is once again twisted. This second twisting transposes the filaments within the segments, thereby producing a superconductor that is resistant to flux jumps induced by self-field losses.

United States Patent Critchlow et a1. I

[ MULTI-FILAMENT COMPOSITE SUPERCONDUCTOR WITH TRANSPOSITION OFFILAMENTS Inventors: Philip R. Critchlow, 1843 Edgewood, St. Bruno,Quebec, Canada; Bruce A. Zeitlin, 234 Henry Ct., North Plainfield, NJ.07060 Filed: Sept. 6, 1972 Appl. N0.: 286,625

US. Cl. 174/126 CP, l74/DIG. 6, 174/114 S, 174/128 Int. Cl. H01v 11/08Field of Search ..l74/l28, 126 CP, 114 S, 174/129, DIG. 6

References Cited UNITED STATES PATENTS 4/1933 Milliken l/l937 Gilbert11/1971 Morton 4/1972 Woolcock..

174/128 X 174/128 X l74/DIG. 6 l74/DIG. 6

[ Sept. 10,1974

3,699,647 10/1972 Bidault l74/DIG. 6

3,702,373 11/1972 Ecomard 174/DIG. 6 FOREIGN PATENTS OR APPLICATIONS708,162 4/1931 France 174/114 S Primary Examiner-E. A. GoldbergAttorney, Agent, or Firm-Larry R. Cassett; I-l.

Hume Mathews [57] ABSTRACT A multi-filament superconducting compositecomposed of a plurality of segments that is intrinsically stable. Theindividual segments are multi-filament composites that have been twistedafter assembly and mechanical working and thereafter subsequentlydeformed. The deformed segments can be triangular or rectangular and areassembled into a second composite. After assembly the second compositeis once again twisted. This second twisting transposes the filamentswithin the segments, thereby producing a superconductor that isresistant to flux jumps induced by selffield losses.

9 Claims, 9 Drawing Figures MULTI-FILAMENT COMPOSITE SUPERCONDUCTOR WITHTRANSPOSITION OF FILAMENTS BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates generally to superconductors and morespecifically to multi-filamentary superconductor composites that areintrinsically very stable.

2. Prior Art A significant factor in the development of superconductorsand superconducting devices is the attainment of adiabatic stability.This is essential in order to attian reliable performance insuperconducting devices. Excellent superconducting properties have beenattained in laboratory testing of short samples. However, whensuperconducting devices were fabricated fromlong superconductor wire,performance comparable to short sample testing was not attained.

Failure to'obtain short sample performance in superconducting deviceshas been attributed, in part, to flux jumps. Flux jumps causedegradation in superconducting devices by creating sudden local releasesof energy resulting in a premature transition to the normal state andthereby preventing reliable attainment of high current densities.

, It is theorized that flux jumps occur when an external field isapplied to asuperconducting device inducing loops of current at thecritical current density which are unstable and may suddenly decayrapidly with a release of energy. This sudden release of energy causes atransition in the superconducting device fromthe superconducting stateto the normal or resistive state thereby causing degradation of thesuperconductor. This sudden reversal can result in serious physicaldamage to the superconducting device.

Various techniques have been developed to improve the stability ofsuperconductors by minimizing degradation caused by flux jumping. In thepaper, Multifilamentary Superconducting Composites, by P. R. Critchlow,E. Gregory and B. Zeitlin, published in Cryogenics, February 1971, pp.3-10, recent developments that have contributed to the production ofstable superconducting composites are discussed. One such development isthe production of superconductors from composites having 50l',000superconducting filaments in a matrix of high purity copper. However,the apparent advantages offered by miltifilamentary composites were notfully realized because these wires behaved in some respects essentiallyvthe same as an equivalent solid single core conductor and the occurrenceof flux jumping was unchanged.

This paper points out a technique whereby flux jumps induced by anexternal field can be reduced and the full advantages ofmultifilamentary composites attained. If the composite is twisted, thewire is effectively cut into lengths equal to half the twist pitch. Thetwisted wire should, therefore, behave as a collection of isolatedfilaments and consequently be more stable than acomparable untwistedmultifilament composite.

Critchlow et al disclose that a twisted composite is still susceptibleto degradation and loss from self-field effects. The self-field of thewire is produced by the transport current flowing in it. This results inan unequal current distribution in which the outer filaments carry'morecurrent than the inner ones. A flux jump I thereby afl'ecting the sizeof the may occur thereby producing more uniform current distributionwith a concomitant transition to the normal state.

Twisting alone will not eliminate self-field effects because thefilaments never change their radial position. Transposition will arrangethe individual filaments so that they occupy successively every positionof the cross-section. The filaments in an individual wire cannot betransposed. However, by winding a number of these wires into a braid orcable, a transposed conductor can be constructed.

By forming abraid. or cable insures that the inner wires get to theoutside of the conductor cross-section. These techniques, however, arenot entirely satisfactory because rigidity and high packing factorcannot be attained. Rigidity of wires in a'superconducting device isnecessary to prevent objectionable wire motion. The shape of braided orcabled super-conducting wire does not produce a high packing factor.This restricts the amount of wire'than can be contained in a magnetmagnet. Accordingly, thepresent invention provides a novelsuperconductor composite comprised of a plurality of segments and methodof making same that utilizes superconducting filaments embedded in anormal matrix. The segments are twisted so as to obviate eddy currenttype losses resulting from external magnetic fields and the individualfilaments are also transposed so that they occupy the same relativeradial position within the superconductor cross-section, thereby alsoavoiding or eliminating self-field losses.

SUMMARY OF THE INVENTION An object of this invention is to provide anintrinsically stable milti-filament superconductor.

A further object of this invention is to provide an intrinsically stablemilti-filament superconductor composed of a plurality of segmentedcomposites.

Still a further object of this invention is to provide a superconductorthat is free from degradation resulting from self-field losses.

Still a further object of this invention is to provide a method forproducing intrinsically stable multifilament super-conductors.

A further object of this invention method for producing filamentsuperconductors segmented composites.

These and other objects are obtained by assembling a first compositecomposed of a plurality of superconducting rods in a norrnal'matrix.This first'assembly is then mechanically worked until the superconductorrods are reduced to a filament of a diameter approximating the desiredfinal diameter. The mechanically reduced composite is then twisted andsegmented. As used herein, the term segmented means to altermechanically the cross-section of the superconductor composite fromround to triangular or rectangular by passingthe composite through aforming device. A second composite of rectangular or circularcross-section is made by assembling a plurality of the previously formedsegments. This second composite is then mechanically worked until thediameter of the filaments is reduced to approximately 0.3 to 0.4 mil.The mechanically worked composite is then twisted a second time.

is to provide a intrinsically stable multicomposed of a plurality of Theindividual segments constitute composites composed of a suitable numberof superconducting filaments in a matrix of normal material having atwist of the necessary pitch. The segments are resistant to flux jumpinginduced by an external magnetic field and are thereby stabilized againstthis form of degradation.- However, the filamentary strands withineachindividual segment will not vary in their radial location from thecenter of such segment and the segment will not avoid instabilityresulting from generated self-fields. The individual segments arecombined to form a second composite. The second composite is thenmechanically worked and twisted to a predetermined pitch. The convolutedfilaments containedwithin each segment of this second composite assume apath of varying radial position along their lengths from the center ofthe final conductor. Such varying distribution of the individualfilaments produces a more uniform sharing of each filament of thecross-section of the composite conductor so that each filament sharessubstantially equally'inthe conducting current. Thus the tendencytowards instability resulting from self-generated fields is avoided.

The advantages of the superconductor of this invention will be apparentfrom the following drawings and detailed descriptions in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS FIG. 1 is a diagrammatictransverse schematic of a first composite of this invention consistingof superconductor rods in a normal matrix.

FIG. 2 is a diagrammatic longitudinal schematic of a first composite ofthis invention showing the position of an outside filament aftertwisting.

FIG. 3 is a diagrammatictransverse schematic of a first composite aftersegmenting.

FIG. 4 is a diagrammatic transverse schematic of another embodiment of afirst composite after segment- FIG. 5 is a diagrammatic transverseschematic of a second composite consisting of a plurality of triangularsegments.

FIG. 6 is a diagrammatic longitudinal schematic of a second compositeshowing the position of a filament within the composite. I

FIG. 7 is a diagrammatic transverse schematic of another embodimentshowing a second composite consisting'of a plurality of rectangularsegments.

FIG. 8 is a diagrammatic transverse schematic of a second compositeafter twisting;

FIG. 9 is a diagrammatic longitudinal schematic of a second compositeshowing the position of a transposed filament within a segment aftertwisting.

DETAILED DESCRIPTION OF THE PREFERRED I EMBODIMENT Referring to FIG. 1,there is shown a superconducting composite 10 consisting of a normal'matrix 12 4 v properties. In this instance composite 10 is reduced to adiameter approximating the desired final diameter.

After fabrication, composite 10 is twisted. As shown in FIG. 2, aftertwisting representative filament 14a follows path 16. The pitch or rateof twist is dependent upon the rate at which the superconducting devicewill be charged. For most applications at pitch of A to 5 twists perinch is acceptable. The intermediate diameter must be of such amagnitude so that reduction to the final diameter will not remove theoriginal twist by I elongating the wire. Generally speaking, thisintermediate diameter must not be more than twice the final desireddiameter.

FIGS.'3 and 4 show two different embodiments, 10a and 10b, of segmentedcomposite 10. After composites 10a or 10b are mechanically worked to anintermediate diameter and twisted, they are then segmented by mechanicalshaping. Shaping to the desired cross-section may be performed by usingdies, grooved rolls or a Turks-head. A Turks-head consists of 4-hardenedsteel rolls set in planes at right angles to each other. The narrow faceof the rolls, as set in the framework, is adjustable on the same planeso that the assembly of the overlapping roll edges facing each otherwill project a contour of the opening so formed, into the desired shapeof the cross-section of the product to be made.

Preferably the angle 0 in FIG. 3 should be a subdivision of 360, ie. 10,12, 15, 20, 24, 30, 45, 60, 72, 90, 120, etc., so that the triangularorrectangular segments 10a or 10b so produced by the Turks-head can beassembled to form a second composite 22 or 30 as shown in FIGS. 5' and7, respectively. After mechanically forming segments 10a or 10b, thesegments can be further treated in order to facilitate assembly ofcomposformed by ultra-sonically welding the segments together.

FIG. 5 shows a second composite 22 composed of triangular segements 20a,20b, 20c, 20d, 20e, 20f, 20g and 20h. The angle 0 formed during drawingfirst composite 10 through the forming dies in 45. As shown in FIG. 6,representative filament 24in segment 20a is transposed along the path26.

FIG. 7 shows another embodiment of a second composite 30 composed ofrectangular segments 32a, 32b, 32c and 32d.

As shown in FIGS. 5 and 7, after assembly the mating segments have acommon junction point, 28 and 34, which is located at the center of thesecond composite. This places some of the filaments which were formerlysituated at the outside of the original composite on the inside or nearjunction point '28 or 34 of the second composite. These filaments wereoriginally twisted and theywill retain their position in the interior ofthe segment cross-section after assembly of the second composite.Furthermore, filaments originally located near the center of thesegments, such as filament 24, will now be located near the outside ofthe second composite. When the second composite is twisted afterassemrate of twist is represented by numerical 36 in FIG. 8.

The segmented composite is now stabilized against selfinduced fluxjumps. Representative filament 24 in segment a is transposed along thepath 27.

Superconducting wire produced by the novel method of this invention hasseveral distinct advantages over superconducting wireproduced byconventional prior four segments and was approximately 20 mil square.The individual segments each contained 360 filaments approximately 0.30.4 mil in diameter.

From the foregoing, it is apparent that by the present invention therehas been provided a particularly advantageous method for producing anovel intrinsically stable superconducting wire. The invention has beendeart methods. Some of the more apparent advantages include:

A solid superconducting wire of equivalent diameter and equalsuperconductor alloy to nonnal material ratio will exhibit more lossesthat a segmented superconductor composite produced by the inventiondisclosed herein. The windings of superconducting devices, such asmagnets, will be more rigid with less wire motion when wound with thisnovel segmented wire as compared to other intrinsically stablesuperconducting wires, such as braid or cable. Furthermore, the shape ofthis wire produces a higher packing factor than braid or cable,resulting in more wire per superconducting device. Still further, acomposite with a like number of small diameter filaments merely twistedand not transposed will not be stable against self-induced losses andthe advantages of small diameter filaments will not be fully realized.

. SPECIFIC EXAMPLE First Composite A first composite was assembled byinserting 360 rods of a Nb-Ti alloy (Containing 55 percent by weight Nb)into an 8 inch diameter copper billet. This composite was thenmechanically worked until the diameter of the composite wasapproximately 11 mil.

The mechanically worked composite was then twisted. The rate of twistwas from about turns to 5 turns per linear inch Segmenting FirstComposite The 11 mil composite was then passed through a Turks-head. Therolls of theTurks-head were adjusted to produce a square wireapproximately 10 mil square.

Second Composite The square segment was then passed through a bath ofsilver-tin solder (3 percent Ag, 97 percent Sn). A second composite wasformed by paying out four solder-coated segments from four bobbins. Thiscomposite 30 is shown in FIG. 7. The payed out segments were passedthrough a series of closely-spaced aligning dies. The properly alignedsegments then passed through a tapered die wherein a square compositewas formed. At some convenient location heat was applied to thecomposite, melting the solder and bonding the segments together. Theformed composite was twisted at a rate from about A turns to about 5turns per linear inch.

The finished intrinsicially stable composite contained scribed withreference to a presently preferred embodiment, however, it' is intendedto cover such modifications as fall within the spirit and the scope ofthe invention as hereinafter claimed.

We claim:

1. A superconductor comprising:

a plurality of initially twisted mated segments twisted together to forman assembly wherein each said segment consists of a matrix of normalmaterial and a plurality of super-conducting filaments in twisted arraythe filaments within each segment occupy positions near the surface andinterior of said matrix cross-section and are also disposed in a path ofvaring radial position in the longitudinal direction of saidsuperconductor.

2. A superconductor as recited in claim 1 wherein said mated segmentshave a common juncture at the center of said super-conductor.

3. An intrinsically stable superconductor composite that is resistant toflux jumps induced by self-field losses comprising:

a plurality of initially twisted mated segments wherein each saidsegment further comprises a matrix of normal material and superconductorfilaments embedded in said matrix, said segments further containing ahelical twist of a pitch from about /2 to about 5 convolutions perlinear inch.

4. A superconductor as recited in claim 3 wherein said mated segmentsare generally triangular in crosssection.

5. A superconductor as recited in claim 3 wherein said mated segmentsare generally rectangular in crosssection.

6. An intrinsically stable superconductor composite that is resistant toflux jumps induced by self-field losses comprising:

a matrix of normal material; and a plurality of filaments of asuperconducting alloy embedded in said matrix wherein the individualfilaments occupy positions near the surface and interior of said matrixcross-section and are also disposed in a path of varying radial positionin the longitudinal direction of said composite.

7. A superconductor as recited in claim 6 wherein said normal materialis copper.

8. A superconductor as recited in claim 7 wherein said superconductoralloy is a Nb-Ti alloy.

9. A superconductor comprising:

an assembly of initially twisted mated segments wherein each segmentcomprises a matrix of normal material and a plurality of filamentsembedded in matrix, wherein the filaments within each segment occupypositions near the surface and interior of said matrix cross-section andare also disposed in a path of varing radial position in thelongitudinal direction of said superconductor.

1. A superconductor comprising: a plurality of initially twisted matedsegments twisted together to form an assembly wherein each said segmentconsists of a matrix of normal material and a plurality ofsuper-conducting filaments in twisted array the filaments within eachsegment occupy positions near the surface and interior of said matrixcross-section and are also disposed in a path of varing radial positionin the longitudinal direction of said superconductor.
 2. Asuperconductor as recited in claim 1 wherein said mated segments have acommon juncture at the center of said super-conductor.
 3. Anintrinsically stable superconductor composite that is resistant to fluxjumps induced by self-field losses comprising: a plurality of initiallytwisted mated segments wherein each said segment further comprises amatrix of normal material and superconductor filaments embedded in saidmatrix, said segments further containing a helical twist of a pitch fromabout 1/2 to about 5 convolutions per linear inch.
 4. A superconductoras recited in claim 3 wherein said mated segments are generallytriangular in cross-section.
 5. A superconductor as recited in claim 3wherein said mated segments are generally rectangular in cross-section.6. An intrinsically stable superconductor composite that is resistant toflux jumps induced by self-field losses comprising: a matrix of normalmaterial; and a plurality of filaments of a superconducting alloyembedded in said matrix wherein the individual filaments occupypositions near the surface and interior of said matriX cross-section andare also disposed in a path of varying radial position in thelongitudinal direction of said composite.
 7. A superconductor as recitedin claim 6 wherein said normal material is copper.
 8. A superconductoras recited in claim 7 wherein said superconductor alloy is a Nb-Tialloy.
 9. A superconductor comprising: an assembly of initially twistedmated segments wherein each segment comprises a matrix of normalmaterial and a plurality of filaments embedded in matrix, wherein thefilaments within each segment occupy positions near the surface andinterior of said matrix cross-section and are also disposed in a path ofvaring radial position in the longitudinal direction of saidsuperconductor.